1. Field of the Invention
The present invention relates to a device for combining at least two data signals having an input data rate into a single data stream having an output data rate being higher than the input data rate for transmission on a shared medium or vice versa. Particularly, the present invention relates to a single SDH/SONET framer capable of handling a large range of SDH/SONET frames from STM-i to STM-j with an aggregated total capacity corresponding to an STM-j frame where i and j are integers in the range from 1 to 64 or higher according to the STM-N definition of the SDH/SONET standards. More over, the present invention can also be extended to work with STS-1 as lowest range. STS-1 exists in SONET only not SDH and corresponds to a data rate of 51.84 Mb/s a third of the 156 Mb/s of STM-1.
2. Description of the Related Art
The American National Standards Institute has established a new basic standard for high-speed, multiplexed digital data transmission. This is the “synchronous optical network” standard, henceforth referred to as SONET. The SONET standard specifies optical interfaces, data rates, operation procedures and frame structures for multiplexed digital transmission via fiber optic networks.
The International Telecommunications Union (ITU) has adopted the interface principles of SONET and recommended a new global transmission standard for high-speed digital data transmission. This standard is the “synchronous digital hierarchy” (SDH).
For an account of the SDH standard on the “General Aspects of Digital Transmission Systems”, reference is made to the ITU standards documents G.707 (Synchronous Digital Hierarchy Bit Rates), G.708 (Network Node Interface for the Synchronous Digital Hierarchy), G.709 (Synchronous Multiplexing Structure), G.782 (Types and General Characteristics of Synchronous Digital Hierarchy (SDH) Equipment), and G.783 (Characteristics of Synchronous Digital Hierarchy (SDH) Equipment Functional Blocks), all issued March 1993.
The SDH standard is designed to enable manufacturers to develop telecommunications equipment which: a) will be interchangeable in all telecommunication networks built around the world to its standard; and which b) is backwards compatible, i.e. can be used with data which is in the older telecommunications formats used in North America, Europe and Japan.
This is achieved by a hierarchy of so-called “Containers” (C) and “Virtual Containers” (VC), see FIG. 1. The containers, e.g. C-4, C-3, C-12, etc., are information structures designed to accommodate data traffic with specific transmission rates. The C-4 container carries traffic with a base rate of up to 139 264 kbit/s, C-3 carries either up to 44 736 or 34 368 kbit/s, etc. The containers are turned into virtual containers by adding “Path Overhead” information (POH) to it. By procedures defined as multiplexing, mapping, or aligning, data structures are generated which are constitutive to the SDH. These data structures are named “Administrative Unit Groups” (AUG) and “Synchronous Transport Module” (STM). The label of an STM is defined by the number of AUGs it carries: a STM-4 contains for example four AUGs. An AUG contains either one “Administration Unit” (AU) of type 4 or three AU-3. Referring to the simplest case, in turn one AU-4 contains one C-4 signal and one AU-3 carries one C-3 signal.
The SDH/SONET data frames, i.e., the STM-N signals, are 125 micro-seconds long. The amount of data transmitted in each frame depends on the hierarchy level N of the signal.
The higher hierarchical levels are transmitted at higher data rates than the basic STM-1 level of approximately 155 Mbit/s. (The exact transmission rate is defined as 155.52 Mbit/s. However here and in the following transmission rates are often denoted by their approximate values. This in particular due to the fact that the exact data transmission rates are distorted by overhead data traffic and idle cell stuffing.) The integer N indicates how many times faster the data is transmitted than in the STM-1 level.
For example STM-4 denotes a data transmission rate of 622 Mbit/s, whereby each data frame contains four times as many bytes as does a frame of STM-1. Currently, the highest defined SONET/SDH level is STM-256/STS-768 which has a data rate of 39.81312 Gb/s. Clearly, each part of the STM-N signal is broadcast in the same time as the corresponding part of an STM-1 signal, but contains N times as many bytes.
The STM-1 signal, as shown in FIG. 2, contains an information rectangle of 9 rows with 270 bytes/row corresponding to a SONET/SDH data rate of 155.52 Mbit/s. The first 9 bytes/row represent the “Section Overhead”, henceforth SOH. The remaining 261 bytes/row are reserved for the VCs, which in FIG. 1 is a VC-4. The first column of a VC-4 container consists of the “Path Overhead” (POH). The rest is occupied by the payload (a C-4 signal). Several VCs can be concatenated to provide a single transmission channel with a corresponding bandwidth. For example, four VC-4 in a STM4 signal can be concatenated to form a single data channel with approximately 600 Mbit/s capacity: in this case the four VC are referred to in the standard terminology as VC-4-4c and the signal as STM-4c.
This flexibility of the SDH standard is partly due to the pointer concept. In SDH, the frames are synchronized, but the VCs within them are not locked to the frames. So the individual containers of the SDH signals do not have to be frame aligned or synchronized among each other. A “pointer” is provided in the Section Overhead which indicates the position of the above introduced POH, i.e. the start of a virtual container in the SDH frame. The POH can thus be flexibly positioned at any position in the frame.
The multiplexing of information into higher order SDH frames becomes simpler than in the old data standards, and an expensive synchronization buffer is not required in SDH.
Similarly, lower order signals can be extracted out of and inserted into the higher order SDH signals without the need to demultiplex the entire signal hierarchy. The pointers are stored in the fourth row of the Section Overhead.
The Section Overhead is further subdivided into: 1. The “Regenerator Section Overhead” or RSOH. This contains bytes of information which are used by repeater stations along the route traversed by the SONET/SDH Signal.
The Regenerator Section Overhead occupies rows 1–3 of the Section Overhead.
The “Multiplexer Section Overhead” or MSOH. This contains bytes of information used by the multiplexers along the SONET/SDH signal's route. The Multiplexer Section Overhead occupies rows 5–9 of the Section Overhead. These sections are assembled and dissembled at different stages during the transmission process. FIG. 2 also shows an exploded view of the MSOH.
In the SONET system, a base signal of 51.84 Mbit/s is used. It is called the Synchronous Transport Signal level 1, henceforth STS-1. This has an information rectangle of 9 rows with 90 bytes/row. The first three bytes/row are the section overhead and the remaining 87 bytes/row are the “Synchronous Payload Envelope”, henceforth SPE.
Three of these SPEs fit exactly into one Virtual Container-4. Thus signals in the STS-1 signal format can be mapped into an STM-1 frame. Furthermore, frame-aligned STS-1 or STM-1 signals can be multiplexed into higher order STM-N frames.
In general, any lower data rate signal which is combined with other such signals into new data frames of higher rate is referred to as a “tributary” signal. For example in the previous paragraph, the three STS-1 signals which are combined into one STM-1 signal are tributary signals.
Digital Cross-Connect (DCC) functionality provides the possibility of rearranging the temporal (in case of a serial high-rate signal) or the spatial (in case of a demultiplexed high-rate signal) order of the low-rate signals or tributaries within the high-rate signal.
Add/Drop functionality allows to extract and/or replace one or more tributary signals from the high-rate signal. It is also known as Drop/Insert functionality.
From WO 98/26531 a digital cross connect and add/drop multiplexing device for SDH or SONET signals is known. In the device four modules for transmitting and four modules for receiving SDH/SONET data traffic are cast together into one chip die forming a STM-4 chip, since the four modules each output a STM-1 signal. Four of these STM-4 chips, operating in parallel, are able to process a STM-16 signal. Each of the four STM-4 chips is connected with four exterior interfaces to a multiplexer/demultiplexer unit. Each of these exterior interfaces is coordinated with one of the basic modules on the respective chip. The SONET/SDH signals transmitted on network lines are transferred out of and into the multiplexer/demultiplexer unit. Four interior interfaces of each STM-4 chip carry data signals extracted from, or to be inserted into the SONET/SDH frames. Depending on the interconnections of the interior interfaces either digital cross-connection or local add/drop functions are provided.